1. Field of the Invention
The present invention relates to a display element, a method for driving the same, and an information display system including the same.
2. Description of the Related Art
Recently, development of electronic paper is active in enterprises and universities. Promising applications of electronic paper include electronic books first of all, sub-displays of mobile terminals, and display portions of IC card. Writing of electronic paper on a wireless basis is studied as a promising applied technique of electronic paper. Existing interfaces available for wireless writing include wireless LANs, Bluetooth (a registered trademark), and contactless IC card systems. When a wireless LAN or Bluetooth is used, electronic paper must be provided with a battery. In the case of a contactless IC card system, data in an IC card are read and written using electric waves transmitted from a reader/writer as a temporary power source. Since the field strength is several tens of milliwatt, the use of a contactless IC card system can make it possible to provide a wireless and battery-less driving system in which electronic paper is written for display without providing it with a battery.
Display elements advantageously used in electronic paper include display elements using a cholesteric liquid crystal. Display elements using a cholesteric liquid crystal allows a reduction in power consumption which is a prerequisite for a wireless and battery-less driving system because they have memory characteristics to allow display to be held semi-permanently. Display elements using a cholesteric liquid crystal also have excellent characteristics, i.e., high color display characteristics which allow vivid display of colors, high contrast, and high resolution. A cholesteric liquid crystal can be obtaining by adding a relatively large amount (several tens %) of a chiral additive (chiral material) to a nematic liquid crystal, and it is therefore sometimes called a chiral nematic liquid crystal. A cholesteric liquid crystal forms a cholesteric phase in which molecules of the nematic liquid crystal are helically (spirally) aligned.
A display element using a cholesteric liquid crystal displays is enabled for display by controlling the state of alignment of liquid crystal molecules. States of alignment of a cholesteric liquid crystal include a planer state in which incident light is reflected and a focal conic state in which incident light is transmitted. Those states exist with stability even when there is no electric field. A liquid crystal layer in the focal conic state transmits light, and a liquid crystal layer in the planar state selectively reflects light having particular wavelengths in accordance with the helical pitch of liquid crystal molecules.
FIG. 12A schematically shows a configuration of a common liquid crystal display element, and FIG. 12B schematically shows a configuration of a liquid crystal display element using a cholesteric liquid crystal. As shown in FIG. 12A, a common liquid crystal display element has one liquid crystal display layer 101 in which pixels of red (R), green (G), and blue (B) are arranged side by side. On the contrary, as shown in FIG. 12B, a liquid crystal display element using a cholesteric liquid crystal generally has a structure formed by stacking three liquid crystal display layers 101R, 101G, and 101B in which pixels of the colors R, G, and B are provided, respectively. The liquid crystal display layers 101R, 101G, and 101B are enabled for display of the colors R, G, and B by varying the helical pitch of liquid crystal molecules. A liquid crystal display element using a cholesteric liquid crystal has an aperture ratio which is about three times greater than that of a common liquid crystal display element. A liquid crystal display element using a cholesteric liquid crystal is therefore capable of color display with high brightness because it has light utilization efficiency (reflectance) that is about three times higher than that of a common liquid crystal display element.
Patent Document 1: Japanese Utility Model No. 3089912
Patent Document 2: JP-A-2003-66413
Patent Document 3: JP-A-2002-108308
However, a liquid crystal display element for color display using a cholesteric liquid crystal suffers from a significant increase in power consumption when display is rewritten because it has a structure formed by stacking three monochromatic liquid crystal display elements and requires a driving voltage of ten and a few V or more. In the case of a wireless and battery-less driving system using feeble electric waves as power, an increase in a communication distance results in a reduction in receiving power. Therefore, a liquid crystal display element employing a wireless and battery-less driving system has a problem in that it is liable to operational failures attributable to shortage of power.